How to achieve high precision PCB circuit board
The high precision of the circuit board refers to the use of fine line width / spacing, micro holes, narrow ring width (or no ring width), and buried and blind holes to achieve high density. And high precision means that the result of "thin, small, narrow, thin" will inevitably bring about the requirement of high precision. Take the line width as an example: O. Line width of 20mm, produced according to regulations. 16 ~ 0.224mm is qualified, the error is (O.20 soil 0.04) mm; and O. For the line width of 10 mm, the error is (0.10 ± O.02) mm. Obviously, the accuracy of the latter is increased by a factor of two, and it is not difficult to understand by analogy, so the requirement of high precision is not discussed separately. But it is a prominent problem in production technology.
(1) Fine wire technology future high fine wire width / spacing will be from 0.20mm-O. 13mm-0.08mm-0.005mm, in order to meet the requirements of SMT and multi-chip packaging (Multicchip Pack, MCP). Therefore, the following techniques are required.
①Using thin or ultra-thin copper foil (<18um) base material and fine surface treatment technology.
②Using thinner dry film and wet lamination process, thin and good quality dry film can reduce line width distortion and defects. The wet film can fill a small air gap, increase the adhesion of the interface, and improve the integrity and accuracy of the wire.
③ Adopt electrodeposition photoresist film (Electro-deposited photoelectric, ED). Its thickness can be controlled in the range of 5 ~ 30 / um, which can produce more perfect fine wires. It is especially suitable for narrow ring width, no ring width and full plate plating. At present, there are more than ten ED production lines in the world.
④Adopt parallel light exposure technology. Since the parallel light exposure can overcome the influence of the line width variation caused by the oblique light of the "point" light source, a fine wire with accurate line width and smooth edges can be obtained. However, the parallel exposure equipment is expensive, requires high investment, and requires working in a high-cleanness environment.
⑤Using automatic optical detection technology (Automatic Inspection, AAOI). This technology has become an indispensable means of detection in the production of fine wires, and is being rapidly promoted, applied and developed. For example, AT & T has 11 AoIs, and TDCO has 21 AoIs specifically for detecting the inner graphics.
(2) Micro-hole technology The functional holes of the printed board for surface mounting are mainly used for electrical interconnection, which makes the application of micro-hole technology more important. The use of conventional drill bit materials and CNC drilling machines to produce micro holes has many failures and high costs. Therefore, the high-density printed circuit boards are mostly made by the finer wires and pads. Although great results have been achieved, their potential is limited. To further improve the density (such as wires less than 0.08 mm), the cost is urgent. Liters, and therefore turned to micropores to improve the densification.
In recent years, breakthroughs have been made in the technology of CNC drilling machines and micro-bits, so the micro-hole technology has developed rapidly. This is the main outstanding feature in the current PCB production. In the future, the technology of forming micro-holes will mainly rely on advanced CNC drilling machines and fine micro-heads. The small holes formed by laser technology are still inferior to the small holes formed by CNC drilling machines from the viewpoint of cost and hole quality.
① CNC drilling machine At present, CNC drilling machine technology has made new breakthroughs and progress. And formed a new generation of CNC drilling machine characterized by drilling tiny holes. The efficiency of micro-hole drilling machine for drilling small holes (less than 0.50 mm) is 1 times higher than that of conventional CNC drilling machines, with fewer failures, and the rotation speed is 11 ～ 15r / min; 1 ～ 0.2mm micro-hole, using high-quality small bit with high cobalt content, can be stacked with three plates (1.6mm / piece) for drilling. When the drill bit is broken, it can automatically stop and report the position, automatically replace the drill bit and check the diameter (the tool library can accommodate hundreds of pieces), and can automatically control the constant distance and drilling depth of the drill tip and the cover plate, so that blind holes can be drilled , Will not drill the table. The table of CNC drilling machine adopts air cushion and magnetic levitation type, which moves faster, lighter and more accurately without scratching the table. Such drilling machines are currently very popular, such as the Mega 4600 from Pruit in Italy, the Excel Ion 2000 series in the United States, and new generation products such as Switzerland and Germany.
② There are indeed many problems with laser drilling conventional CNC drilling machines and bits to drill micro holes. It has hindered the progress of micro-hole technology, so laser erosion has received attention, research and application. However, there is a fatal flaw, that is, the formation of a horn hole, which becomes more serious as the thickness of the board increases. Coupled with the problems of high temperature ablation pollution (especially multi-layer boards), the life and maintenance of the light source, the repeat accuracy of the etched holes, and the cost, the promotion and application of micro holes in the printed board is limited. However, laser etched holes are still used in thin high-density micro-well plates, especially in MCM-L high-density interconnect (HDI) technology, such as M.I. C. The high-density interconnection combining the etched holes of polyester film and metal deposition (sputtering technology) in MS. The formation of buried holes in high-density interconnected multilayer boards with buried and blind hole structures can also be applied. However, due to the development and technological breakthroughs of CNC drilling machines and micro-drills, they were quickly promoted and applied. Laser drilling on the surface
Applications in mounting circuit boards cannot form a dominant position. But there is still a place in a certain area.
③ buried, blind, through-hole technology buried, blind, through-hole combination technology is also an important way to improve the density of printed circuits. Generally, the buried and blind holes are tiny holes. In addition to increasing the number of wiring on the board, the buried and blind holes use the "closest" inter-layer interconnection, which greatly reduces the number of through holes formed, and the isolation plate setting will also be greatly Reduction, thereby increasing the number of effective wiring and interlayer interconnections in the board, and increasing the density of interconnections. Therefore, the multi-layer board combined with buried, blind, and through holes is at least 3 times higher than the conventional full through hole board structure under the same size and number of layers. If under the same technical indicators, buried, blind, The size of the printed board combined with through holes will be greatly reduced or the number of layers will be significantly reduced. Therefore, in high-density surface-mounted printed boards, buried and blind hole technologies are increasingly used, not only in surface-mounted printed boards in large computers and communication equipment, but also in civil and industrial applications. It has also been widely used in the field, even in some thin boards, such as various types of PCMCIA, Smart, IC card and other thin six-layer boards.
The printed circuit boards with buried and blind hole structures are generally completed by the "sub-board" production method, which means that it can be completed after many pressing plates, drilling, hole plating, etc., so precise positioning is very important .